Chemical Energetics Chapter 9, presented by Coco Chan TABLE OF CONTENTS Key OBJECTIVES Be able to describe what is meant by exothermic and endothermic reactions • • • • • • • Be able to interpret energy level diagrams showing exothermic and endothermic reactions Describe the release of heat energy by burning fuels Know that hydrogen is used as a fuel Know that radioactive isotopes, such as 235π, can be used as a source of energy Know that bond breaking is an endothermic process and bond formation is an exothermic process Be able to draw and label energy level diagrams for exothermic and endothermic reactions using data provided Be able to calculate the energy change in a reaction using bond energies • Energy Change • • • • There is always an energy change during a chemical reaction. Also known as enthalpy change ΔH Chemical reaction: breaking and reforming bonds Exothermic reaction: give out energy Endothermic reaction: take in energy What happens in a chemical reaction? Chemical reactions are like building lego blocks! Car ο House 1. Take apart the block 2. reassemble in a different way What happens in a chemical reaction? Step 1 Bond-breaking Step 2 Bond-forming Energy changes during a chemical reaction: Step 1: during bond-breaking, energy is needed (heat is absorbed) - endothermic Step 2: during bond-forming, energy is given out (heat is released) - exothermic In an exothermic reaction: Heat energy given out to the surrounding, so the temperature we measured increases. • • • Combustion of fuels (including respiration) Many oxidation reactions Neutralisations In an endothermic reaction: Heat energy is taken from the surroundings, so the temperature decreases • • Thermal decomposition photosynthesis Energy Level Diagram Diagrams showing the enthalpy change during an exothermic and endothermic reaction Test yourself 1. A student adds a small piece of sodium into a beaker of water. The temperature of the water increases from 21οΈC to 39οΈC. Which type of reaction (exothermic or endothermic) has occurred and why? 2. Two steps are needed, to go from reactants to products in a chemical reaction. What are they? Drawing energy profile diagram β β Draw the horizontal and vertical axes. Label the axes. Draw two bars (different heights): β Endothermic – reactant < product Bond breaking – energy is taken in, arrow always points upward Bond forming – energy is given out, arrow always points downward β β β exothermic – reactant > product Bond breaking and bond forming We know that when a reaction takes place, bonds are broken, and new bonds formed. What bonds are broken and what bonds are formed during the following reaction? π»2 + πΆπ2 2 π»πΆπ Activation energy is the energy required to break all the necessary bonds in the reactants. The stronger the bonds are, the more energy is needed to break them. Calculating the energy changes in reactions The molar enthalpy change for reactions can be calculated: π₯π» = energy taken in – energy given out Consider reaction between hydrogen and chlorine again: 1 H-H bond is broken: 1 Cl-Cl bond is broken: Total energy taken in = 2 H-Cl bonds are made: Total energy given out = Overall, there is less energy taken in than given out. Therefore, overall energy is being released – this reaction is exothermic. 3. Identify the type of reaction from the following energy level diagram. 4. Hydrogen Bromide decomposes to form hydrogen and bromine. 2π»π΅π π»2 + π΅π2 Calculate the enthalpy change and identify the type of reaction. Going over some important terms Exothermic reaction β Energy given out to the surroundings β Surroundings become hot β Bond making - exothermic Activation energy Activation energy is the energy required to break all the necessary bonds in the reactants. Endothermic reaction β Energy is taken from the surroundings β Surroundings become cold β Bond breaking - condothermic Activation Energy Ea ζ΄»εθ½ Energy needed to break all the bonds in the reactants – minimum energy needed to start off a reaction. If Ea is low, not much energy is needed. For example, mixing water with calcium oxide, the reaction can start spontaneously. • If Ea is high, some reaction requires extra energy to start off. For example, the reaction between iron and sulfur, requires heat. Then the energy given out can support the reaction to break further bonds. • But for endothermic reactions, for example the decomposition of calcium carbonate, you must continue heating until the reaction is complete. • Energy from fuel • • • A fuel is any substance we use to provide energy. E.g. fossil fuel, hydrogen, ethanol Exothermic or endothermic? What makes a good fuel? οΌ οΌ οΌ οΌ οΌ The amount of energy is given out Availability Be a liquid at room temperature Affordability Environmental friendly Hydrogen as a fuel Hydrogen is used in rocket engines and in fuel cells to power some cars Advantages: • It releases more energy per kilogram than any other fuel (except for nuclear fuels) • It does not pollute as it only produces water on combustion, no other product is formed Disadvantages: • Expensive to produce and requires energy for the production process • Difficult and dangerous to store and move around (usually stored as liquid hydrogen in highly pressurised containers) Hydrogen fuel cell • • • • A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode These cells are becoming more common in the automotive industry to replace petrol or diesel engines H2 and O2 are pumped through two porous electrodes where the halfreactions occur The following reaction occurs at the anode: Radioactive isotopes as fuel Uranium-235 undergoes decay and gives off heat energy which nuclear power stations harness The heat it produces is used to heat water to steam, which in turn is used to power turbines to generate electricity Nuclear fuel energy is clean as it does not produce pollutants such as CO2 or oxides of nitrogen or sulfur But nuclear power plants are expensive to build and maintain as well as being potentially dangerous in the event of an accident as radioactive materials may be released SCHEDULE APRIL 30 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 TABLE TEST 1 Describe here your metrics 1 Describe here your metrics 2 Describe here your metrics 3 Describe here your metrics 4 Describe here your metrics 5 TEST 2 TEST 3 TEST 4 THANKS Does anyone have any questions? addyouremail@freepik.com +91 620 421 838 yourcompany.com Fonts & colors used This presentation has been made using the following fonts: Anton (https://fonts.google.com/specimen/Anton) Advent Pro (https://fonts.google.com/specimen/Advent+Pro) #2d406a #30a0a0 #fcbf4a #f3f3f3 Use our editable graphic resources... 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